Tmux Enter Key Handling
What This Solves
When sending input to tmux panes running interactive applications (Claude Code, Codex, vim, etc.), the Enter keyword in tmux send-keys fails silently. Text appears but is never submitted.
Working Approach
Send Enter as hex 0x0d (carriage return) in a separate command:
Bash
# ā
CORRECT - send text, then Enter as hex
tmux send-keys -t "session-name" "Hello world"
tmux send-keys -t "session-name" -H 0d
Python
import subprocess
# Send text
subprocess.run(['tmux', 'send-keys', '-t', session, prompt])
# Send Enter as hex (REQUIRED for TUIs)
subprocess.run(['tmux', 'send-keys', '-t', session, '-H', '0d'])
Other Special Keys
| Key |
Hex Code |
Usage |
| Enter |
0d |
tmux send-keys -t sess -H 0d |
| Escape |
1b |
tmux send-keys -t sess -H 1b |
| Tab |
09 |
tmux send-keys -t sess -H 09 |
Failed Attempts
ā ļø Read this first - These approaches don't work:
| Attempt |
Why It Failed |
Time Wasted |
tmux send-keys -t sess "text" Enter |
Enter keyword ignored by TUI apps |
2+ hours debugging |
subprocess.run([..., prompt, 'Enter']) |
Same issue - Python passes it but TUI ignores |
1 hour |
| Checking return code for success |
returncode=0 means tmux ran, NOT that input was submitted |
30 min false confidence |
Verification (Critical!)
Never trust return codes. Always verify visually:
# Send prompt
tmux send-keys -t sess "Write a haiku"
tmux send-keys -t sess -H 0d
# Wait for response
sleep 5
# VERIFY the response actually appeared
tmux capture-pane -t sess -p -S -50
Only report success when you SEE:
- The prompt text in output
- Processing indicator (e.g., "Thinking...")
- Actual response content
- New input prompt ready
Environment & Dependencies
- Applies to: Any tmux version
- Affected apps: Claude Code, Codex CLI, vim, emacs, any ncurses/TUI app
- Works fine with: Regular shell prompts (bash, zsh) - but hex is safer everywhere
Session Reference
- Date: 2024
- Source: tmux_sdk development, multiple debugging sessions
- Original issue: Commands appeared typed but never executed
